An urban terrorist attack involving release of biological warfare agents such as bacillus anthracis (anthrax) is presently a realistic concern. Weaponized anthrax spores are extremely dangerous because they can gain passage into the human lungs. A lethal inhalation dose of anthrax spores for humans, LD50 (lethal dose sufficient to kill 50% of the persons exposed) is estimated to be 2,500 to 50,000 spores (see T. V. Inglesby, et al., “Anthrax as a Biological Weapon”, JAMA, vol. 281, page 1735, 1999). Some other potential weaponized bio-agents are yersinia pestis (plague), clostidium botulinum (botulism), and francisella tularensis. In view of this potential threat, there is currently a need for an early warning system to detect such an attack.
Laser particle counters are known in which a laser beam is directed through a sample and the light, which travels through the sample, is detected and analyzed to detect scattered light from particles in the sample. One problem with existing detectors or particle counters which, are designed for detection of scattered light is that the scattering signal must be extracted from the incident illumination light source signal. This involves detecting a weak signal (scattering from small particles) from a very noisy background (glare from the laser source). Currently, conventionally designed laser particle counters are fragile and expensive, and unsuited to this application. The conventional techniques used for laser particle counting include the laser Doppler method, which measures the speed of the particle and deduces size information, the transient time method which measures the time needed for particles to traverse a sensing region, and large angle multi-sensor design, which is capable of measuring only small particles. A proposed bio-sensor based on laser-induced fluorescence using a pulsed UV laser is described by T. H. Jeys, et al., Proc. IRIS Active Systems, Vol. 1, p. 235, 1998. This is capable of detecting an aerosol concentration of five particles per liter of air, but involves expensive and delicate instruments. See also Hairston et al., J. Aerosol Sci., Vol. 28, No. 3, p. 471-482 (1997). Other particle counters are manufactured by Met One Instrument, Inc, of Grants Pass, Oregon, Particle Measurement Systems, Inc., of Boulder, Colo., and Terra Universal Corp., of Anaheim, Calif. By virtue of their design, these particle counter configurations require precision optical alignment, as well as sophisticated sensors and electronics. These products are geared towards laboratory use and cost thousands of dollars for a single unit. Thus, they are not suitable for a field-deployed detector, nor are they designed specifically for detection of biological warfare agents.
Various detectors have been designed to detect airborne allergen particles and provide warning to sensitive individuals when the number of particles within an air sample exceeds a predetermined minimum value. These are described in U.S. Pat. Nos. 5,646,597, 5,969,622, 5,986,555, 6,008,729, and 6,087,947, all of Hamburger et al. These detectors all involve direction of a light beam through a sample of environmental air such that part of the beam will be scattered by any particles in the air, a beam blocking device for transmitting only light scattered in a predetermined angular range corresponding to the predetermined allergen size range, and a detector for detecting the transmitted light. An alarm is actuated if the light detected at the detector is above a predetermined level. Although these devices are sufficient for the purpose of providing an alarm indication based on the presence of allergen particles, they are not suitable for field deployment and do not meet the more stringent requirements for a pathogen detector for detecting biological warfare agents.